Gullies and Newly Identified Flow Features in Same Mars Crater
This image contrasts gullies and recurring warm-season slope flows appearing in the same crater, in the middle southern latitudes of Mars. It was taken Nov. 27, 2007, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.Gullies are on the pole-facing slope, which is on the northern side of this crater (upper arrow). Recurrent warm-season flows are on the equator-facing wall (lower arrow).Gullies are typically much larger than the warm-season flows. As seen in this crater, gullies have a typical range of features including alcoves, channels and aprons.The flow features are narrow (one-half to five yards or meters wide), relatively dark markings on steep (25 to 40 degree) slopes at a few other southern hemisphere locations. Repeat imaging by HiRISE shows the features appear and incrementally grow during warm seasons and fade in cold seasons. Sequences of observations recording the seasonal changes in these flows might be evidence of salty liquid water active on Mars today.

ESA's Mars Express has released images showing an unusual accumulation of young craters in the large outflow channel called Ares Vallis while older craters have been reduced to ghostly outlines by the scouring effects of ancient water.

The images seem to indicate that over 3.8 billion years ago, large volumes of water must have rushed through the Ares Vallis in a powerful way.

The most prominent crater called Oraibi lies in the channel and is about 32 km across and images show it is filled with sediments and its southern rim has been eroded by water.

NASA's Pathfinder mission landed in this region in 1997, 100 km to the north of the crater and off the right-hand side of this image.

"The great outflow that partially eroded Oraibi also cut stepped riverbanks and excavated parallel channels in the riverbed that indicate the flow path. Streamlined islands have been left standing above the valley floor, again indicating the direction taken by the flow," the ESA web site explains.

The images also show a number of 'ghost craters' which were once fully formed craters, which rims were eroded by water or wind. Those rims later became filled by sediments.

With some of them located on the plateau, researchers say that even that higher ground may have been at least partially overrun by flooding, while they point out that the solitary mounds could represent the remaining sections of the plateau's original surface.

Images also revealed a more recent impact on the martian surface.

Observations led to the discovery of parts of an ejecta blanket , made of material excavated from the ground during the formation of an impact crater and a landslide roughly 4 km wide and probably caused by the same impact can also be observed.

Furthermore, there are numerous small craters in the image, appearing both in clusters and in aligned groups. An abundance of such craters can result when an asteroid or other projectile breaks up into many pieces in the atmosphere before crashing to the ground.

"Clusters of craters may also be created when a large impact ejects rock fragments with such force that they travel from a few kilometres to hundreds of kilometres before returning to the surface, creating new impacts called secondary craters," ESA also says.